The present invention relates to a high pressure tank having a hollow liner and a fiber reinforced resin layer which covers the outer surface of the liner.
Awareness of the need to restrain global warming has been increasing recently. Particularly, developments in fuel-cell electric vehicles and hydrogen-powered vehicles have been made progressively for the purpose of reducing carbon dioxide which is emitted from vehicles. These vehicles induce power by electrochemical reaction of hydrogen and oxygen and supply the power to motors to generate drive force. A hydrogen tank is used as a hydrogen source and is filled with hydrogen at high pressure.
FIG. 16 is a cross-sectional view of a hydrogen tank 51 for use as a high pressure tank disclosed in Japanese Laid-Open Patent Publication No. 2002-181295. The hydrogen tank 51 has a liner 52 having a hollow barrel shape. The liner 52 is made of a material which ensures an airtight condition (e.g., high-density polyethylene). A top boss 53 and an end boss 54 made of aluminum or the like which has high thermal conductance are respectively secured to the front end and rear end of the liner 52. The top boss 53 and the end boss 54 are assembled while partly exposed to the outside. Heat exchange is carried out between the interior of the hydrogen tank 51 and the outside via the top boss 53 and the end boss 54.
A shell 55 is coated on the entire outer surface of the liner 52. The shell 55 is made of a material which can ensure pressure resistance (e.g., fiber reinforced plastics (FRP)). The liner 52 houses a fin assembly 58 including a plurality of fins 56 and a shaft member 57 which supports the fins 56. Both ends of the shaft member 57 are respectively secured to the top boss 53 and the end boss 54. The fin assembly 58 is made of aluminum or the like which has high thermal conductance.
In a case where the liner 52 housing the fin assembly 58 is manufactured integrally, the juncture portions of the liner 52 and the fin assembly 58 are subjected to vacuum brazing and spinning. As the spinning process applies heat to the liner 52, however, the strength of the liner 52 is lowered, making the liner 52 easier to break. This requires that the liner 52 be subjected to a heat treatment again at, for example, 500 degrees. However, the re-heat treatment may cause separation of the brazed portions or a failure in locally disposed seals. It is therefore necessary to prepare a separable liner in the case where the liner is assembled into the fin assembly 58.
FIGS. 17(a) and 17(b) are partial cross-sectional views exemplarily and partially illustrating hydrogen tanks respectively using separable liners 152 and 252. Each of the liners 152 and 252 has a body portion 59 which has an approximately cylindrical shape and a lid 60 which covers the opening portion of the body portion 59. In the case of the liner 152 in FIG. 17(a), an O-ring 62 is disposed on a seal surface 61 which extends in the radial direction of the liner 152 on one of the contact surfaces of the body portion 59 and the lid 60. In the case of the liner 252 in FIG. 17(b), on the other hand, the O-ring 62 is disposed on a seal surface 53 which extends in the axial direction of the liner 252.
In a hydrogen tank where internal pressure becomes high, the gas pressure expands the liner 152, 252 outward in the axial direction or the radial direction. In the case of the liner 152 in FIG. 17(a), therefore, when the lid 60 is pushed outward in the axial direction (in the state indicated by the two-dot chain line) by the internal gas pressure, the O-ring 62 cannot seal between the body portion 59 and the lid 60, causing gas leakage. In the case of the liner 252 in FIG. 17(b), on the other hand, when the body portion 59 is pushed outward in the radial direction (in the state indicated by the two-dot chain line) by the gas pressure, the O-ring 62 cannot seal between the body portion 59 and the lid 60, causing gas leakage.
The shell of the hydrogen tank receives stress in the axial direction and the radial direction of the hydrogen tank. The ratio of the forces acting in the axial direction and the radial direction of the hydrogen tank 51 is one in the axial direction to two in the radial direction. It is therefore most preferable that the reinforced fibers constituting the shell be laid out both in the direction parallel to the axial direction and the circumferential direction. It is however difficult to lay out reinforced fibers in the direction parallel to the axial direction of the hydrogen tank. In this respect, conventionally, reinforced fibers are laid out corresponding to in-plane winding or helical winding with respect to both the axial-directional ends of the hydrogen tank and are laid out corresponding to in-plane winding or to a combination of helical winding and hoop winding with respect to the cylindrical portion (body portion) other than the ends. As it is difficult to repeatedly perform hoop winding on each end wall, the strength in the radial direction is ensured by helical winding. Because helical winding provides lower strength against radial-directional force relative to hoop winding, however, gas leakage occurs.